Majestic Wine's Steve Lewis: 'Customers are shopping more frequently and spending more per bottle.' Photograph: Graham Turner for the Guardian

When you run Britain's biggest independent wine retailer you don't need an excuse to crack open a bottle of champagne, but Christmas in the Lewis household will start with a bottle of Bollinger Grande Année 2000.

"It's stunning quality," says Majestic Wine chief executive Steve Lewis of a champagne that has been celebrated as "richly fruited" with "yeasty notes". Lewis adds: "Someone produced a magnum at dinner last week and it was so good it silenced four people in the wine trade for two minutes – which is quite an achievement."

Perhaps for occasions like this Lewis has memorised his Christmas wine list: the smoked salmon starter will be accompanied by a white burgundy from Christophe Cordier before the family switches to his favourite Kiwi pinot noir (Martinborough Vineyard's 2008 if you are interested) to set off the roast turkey.

It seems consumers are interested. Majestic is the David to the supermarkets' Goliath with profits up 20% at last count as the decision to lower the minimum purchase from a dozen bottles to six resonated with wine lovers.

The previous limit was a hangover of the old Sunday trading laws and Lewis began trials of the change three weeks after he took over as chief executive in 2008. "We had this real gem of a company with something special and different, which is our staff and customer service, but our market share wasn't moving," he says of the decision. When the company investigated why, the feedback from customers was so positive the market research firm redid the survey to check the results. But customers appeared to be put off by the big outlay required and as a result were visiting its stores only three times a year ahead of a special occasion.

"What the move to six bottles has done is make us more accessible to new and existing customers," says Lewis. Despite the far lower hurdle, the average amount customers spend has fallen only 7% to £122. "Customers are shopping more frequently and experimenting, and as a result they are spending more per bottle." It now has nearly 500,000 active customers on its database, up 14% on last year, and its market share has increased from 3.2% to 3.5%. "If you think about how much new space the supermarkets are opening, you have to move at some pace to hold your market share, let alone grow it," he adds.

Like a lot of its staff, Majestic was born in the 1980s – the average age of its employees is 26 – and its Watford headquarters buzzes like a college JCR. Nearly all the staff are graduates and Lewis says it hires people "who look and sound like the sons and daughters of our customers".

It also promotes from within. Lewis started his career there as a trainee manager 25 years ago and has never looked back. "I get bored very quickly and am always looking for the next challenge, but every time that has happened at work I have been promoted." When he joined in 1985, Majestic, which started as a single shop in London's Wood Green, had sales of £5m. Today it has annual sales of more than £230m and 150 shops, as well as plans for 100 more.

Majestic's success sets it apart in the sector. Supermarkets aside, few retailers are making money from selling wine, with off-licences an endangered species. Last year the owner of Threshers fell into administration and Lewis says the economics of selling wine on the high street "just don't add up" any more against the ferocity of supermarket promotions.

The power of the grocers, who speak for 70% of wine sales, means the average bottle of wine sold in Britain costs £4.52. Majestic's competitive strength lies in the fact that its average bottle price is £6.67, making it "desperately important" to vintners with wines retailing at more than £5. Majestic doesn't even sell the biggest selling brands such as Ernest & Julio Gallo, Blossom Hill and Hardys Crest "because our customers don't ask us for them".

"There aren't many people selling wines at £6-£20 a bottle, so it means our buying power is disproportionately strong," Lewis says. "If you take Bordeaux, we have a 12% market share, while in champagne its 10%. Our Australian wine buyer is there right now and his influence is quite disproportionate to our market share."

Majestic's success has ridden the crest of the "foodie" culture. Lewis says a "huge trend" at present is quality sparkling wine, with sales up 20% in the last six month as Prosecco and New Zealand's Lindauer gain currency with drinkers.

Majestic, he says, is typically "three to four years ahead of the curve for wine trends" and reports an industry drive to reduce alcohol levels inflated by the growth of New World wines (which typically have a higher alcohol content as the warmer climate makes the grapes riper): "Growers are trying to make good red wine at 12% rather than 13.5%."

Lewis's enthusiasm for Majestic appears boundless, but he appears to have a surfeit of energy full stop, describing a hectic schedule shuttling between his flat in Marylebone and the house he shares with his wife and three teenage daughters in Hampshire. "I am a very active person." His passion for wine is matched by a love of history, particularly South American, having studied the subject at UCL. "I know more about Argentina than most people would care to."

Lewis sounds as driven in his free time as at work. Indeed, the family sound quite gung-ho: the girls ride while he enjoys yomping in the countryside with their pet German shepherd. They also travel: the Lewis clan is just back from a walking holiday in Majorca (last year it was the Atlas mountains) and plans a Christmas break in Nevis to celebrate his 21st wedding anniversary. The women in his life, he admits, help to keep his feet on the ground: "There are four people in my household that laugh at me to stop me doing anything middle-aged."

There would seem to be worse jobs than running a wine business. Lewis says: "I think it is a very important part of my job description to buy every new wine that we sell and drink it at home." His mantra is "life's too short to drink bad wine", which probably makes his favourite tipple, a Rioja called Prado Enea, a good recommendation.